Flat roof -- covering holes and soft spots?

Hello, I recently purchased a rowhouse in Baltimore City, which I am getting ready to fix up as a rental property. Thus, I am looking for a cheap yet effective option to repair the roof.
Here's the problem -- The original roof is a built up roof. At some time, probably around 10 years ago, rolled roofing was installed over the badly dried out and leaking tar. Whoever installed the rolled roofing was somewhat careless, as they tore or penetrated the roofing material in several places. This of course allowed water to get through to the older tar roof, which we have already established was leaky.
Over the course of 10 years or so, water slowly leaking in rotted out the wood plank sheathing in several areas, resulting in a few soft spots, plus one spot where the roof is sagging badly between the joists. This sag made a constant puddle which found its way through the roofing materials and did some damage to the interior of the property.
I need to fix this roof quickly and inexpensively. The property is in an area where a 1400 sq ft 4-bedroom rowhome is worth around 55-60k, so the property simply isn't worth a large financial investment for a "permanent" repair. My goal is repair it well enough that I will be able to spread a new layer of cold-app roofing tar from a can every 5 years or so and other than that not worry about it.
Here's the best solution I have come up with, and I would appreciate opinions on its reliability and effectiveness. I would like to use pieces of thick plywood screwed down on top of the existing roof to cover the soft or sagging spots, with the plywood extending out onto the sections of roof which are still mechanically sound. Then I would cover the repairs with tar paper, with the tar paper of course extending past the edge of the plywood by several feet. I would seal under the edges of the tar paper with roofing tar, then recoat the entire surface with the cold-app tar.
It seems logical to me that this type of repair would work, but I would like some thoughts from those more experienced than I am. I realize that cutting out the soft sections may be a better option, but this house is almost 100 years old, so I would expect many many layers of tar built up creating a very thick roof to cut through, and then the need to build up on top of the plywood to make it the same thickness of the roof. It seems like just putting plywood right on top would be just as reliable and significantly less work.
Here are my main concerns:
Is it ok to put a tar roof (from a can) on top of rolled roofing?
Is it ok to do a patch repair like this?
If this repair technique is done well, is it reasonable to expect that it will not leak?
Sorry about the long post, and I hope to hear back from you with your thoughts and experiences.
Thanks, -Neil Covington snipped-for-privacy@comcast.net
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Neil wrote:>Hello,

Flat roofs suck. Soft spots on flat roofs indicate rotted or improperly installed sheathing. Screwing some ply to the soft area will only cause a ponding situation next to it, and will leak in the near future. Better to tear off, install new sheathing, and go with higher-tech materials. Sorry. Tom Work at your leisure!
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I dunno, around here a leaky roof when dried will grow a bacteria or fungus called dry rot, and its contagious as all get out to all wood that touches it - spreads and destroys wood suprisingly fast. Call a roofer or two for estimates at least before you attempt to probably extend the inevitable.
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There's probably already too much weight on the roof for the support joists. Adding more will not fix the problem either. You will probably have to replace several damaged joists and you'll certainly have to do a tearoff and replace the decking and roofing. If you don't fix it right, then it will eventually collapse, and you'll be liable to any tenants for the damages/deaths that will cause. Not to mention, most insurance companies are getting very very picky about the condition of the buildings they will insure, and you may not be able to find insurance for it until the repairs are *properly* completed.
Suck it up and do it right. Because you won't even be able to sell the building unless it's repaired properly.
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